Modern mobile telecommunication networks are mainly based on cell based technology. The idea is that the network consists of cells next to each other covering a certain geographical area. More specifically each of the cells comprise a base station, which is a network element offering the wireless connection i.e. radio channel for the mobile terminals camping in that specific cell. Depending on the network technology the mobile terminals are controlled from the network side in such a manner that the mobile terminal when moving in the area does not face any problems to initiate a connection. The mobile terminal gets information from the network on what to measure and under what circumstances it should reselect from serving cell to another cell. Based on this information the mobile terminal determines when to reselect cell and to what cell, without communicating measurements to the network. This phenomenon in which a terminal is moving from one base station to another is known as cell reselection.
A cell reselection is a process for confirming a solid operation between a terminal and network when the terminal is moving in the network. More specifically, the cell reselection is what the terminal does when it is in idle state or in a connected state with no or very little data communication with the network. The mobile terminal communicates with the network by signaling procedures. The terminal can be in Idle, CELL_FACH, CELL_PCH or URA_PCH when performing cell reselection in UTRAN (Universal Terrestrial Radio Access Network).
At cell reselection the network delivers parameters and priorities to the mobile terminals. The terminals use the information, perform measurements and determine to change cell on its own. The terminal decides to what cell to reselect to.
As can be assumed there needs to be a mechanism to control and manage the cell reselection. There needs to be a mechanism for defining a cell, which is to be selected as a next serving cell, when the terminal is moving in the area. The goal in cell reselection process is that a mobile terminal is always served by such a base station, which offers the best radio channel according to terminal needs.
The cell reselection process becomes a challenging task to implement especially, when the terminal is capable of communicating with multiple radio access technologies and in the area where the terminal is camped there are multiple networks based on different radio access technologies available. Thus, the cell reselection process needs to be implemented in such a manner that it serves the general needs of the terminal, but does not bring any unnecessary effects, such as excessive radio channel measuring causing e.g. charging of battery in a mobile terminal to run out too fast.
The cell reselection mechanism is generally based on the procedure in which a terminal makes measurements in relation to characteristics of predefined radio channels by the network. The network defines system information containing at least the carrier frequencies to measure on and possibly what cells to measure on. Cells that belong to the frequency carrier or to the explicit cells that are to be measured are, in general, allowed cells to reselect to and often called neighboring cells. The system information is delivered to mobile terminals by the base station and the system information is specific to that cell. Worthwhile to mention is that the cell reselection criteria are described in a standard corresponding to the radio technology in use.
Cell reselection may be based on different types of reselection. Generally known alternatives are so called intra-frequency, inter-frequency and inter-RAT (Radio Access Technology) cell reselections. Intra-frequency neighbor cell measurements are performed by a mobile terminal when the current i.e. serving and target cell operates on the same carrier frequency. Inter-frequency neighbor cell measurements are performed by the user terminal when the neighbor cell operates on a different carrier frequency, compared to the current serving cell, but the same radio access technology as the current serving cell. Similarly, inter-RAT measurements are performed by the user terminal when the neighbor cell operates on another radio access technology than the current serving cell.
As already disclosed the cell reselection measurements are performed by a user terminal. The user terminal is configured to measure such parameters i.e. characteristics of radio channel of currently serving cell and neighboring cells that are necessary for cell reselection. The network informs the mobile terminal on the characteristics to be measured explicitly or implicitly by sending some threshold parameter that is related to a measure. Typically such characteristics are measures relating to a signal strength and/or signal quality. For instance, in radio access technology known as Universal Terrestrial Radio Access Network (UTRAN; WCDMA) such measure relating to signal strength is known as CPICH RSCP (Common Pilot Channel Received Signal Code Power) and measure relating to signal quality is known as CPICH Ec/No (The received energy per chip of the Common Pilot Channel divided by the power density in the frequency band). Correspondingly, for radio access technology called LTE (3GPP Long Term Evolution) measure relating to signal strength is known as RSRP (Reference Signal Received Power) and measure relating to signal quality is known as RSRQ (Reference Signal Received Quality).
One further aspect relating to cell reselection is that it is possible to indicate a camping priority for each frequency in the system information. This is called absolute priorities. Each frequency carrier indicated in the system information is classified by a parameter describing its priority, e.g. with a number from 0 to 7. The mobile terminal is configured to reselect to a cell that has the highest absolute priority among the cells that fulfills other criteria defined for finding the candidate cells for reselection. If no absolute priority is indicated for any inter-frequency then offset based cell reselection is applied for inter-frequency cell reselection. The offset based reselection simply means an utilization so called offset value in decibels, where the mobile terminal compares all cells with each other including the serving cell. The inter-RAT (E-UTRAN (Evolved UTRAN) and GERAN (GSM EDGE Radio Access Network)) priorities have to be different from the priority of the serving UTRAN cell's frequency. If no absolute priority is indicated for any GERAN frequency then offset based cell reselection is applied for inter-RAT cell reselection to GERAN.
In addition to absolute priorities the network can also support so called dedicated priorities. The core network can provide with an index (called Subscriber Profile Identity (SPID) or RAT/Frequency Selection Priority (RFSP) for a certain terminal to the radio network controller (RNC). The RNC maps the index to a frequency priority list. The RNC provides the UE with its dedicated priority list by dedicated RRC (Radio Resource Control) signaling. The UE uses the dedicated priorities at cell reselection, hence dedicated priorities overrides absolute priorities sent in system information.
A general principle of absolute priority based cell reselection is illustrated in FIG. 1, which may also be applied in mobile terminals using dedicated priorities received from the network. There are shown two exemplified situation, which the terminal may face. First one is that the terminal notices that the signal quality in general in the serving cell is below a threshold level (Threshserving,Low). In such a situation the terminal is allowed to reselect a lower priority cell if the measured value indicating the quality of the radio channel of the lower priority cell is above a threshold (Threshx,Low) set to that lower priority cell. Similarly, in situations where the radio channel quality of the serving cell is good i.e. on the level (SNonIntrasearch), where the terminal searches only higher priority cells, the terminal may reselect to a higher priority cell if a characteristic of a radio channel in the higher priority cell exceeds a threshold (Threshx,High) level set for the higher priority cell. SNonIntrasearch is a level above which only cells with higher priority layers are searched and below which the search is accomplished to cells with any priority.
For determining if the serving cell has an acceptable signal strength level for camping on it, so called S-criteria is used. It basically defines the coverage of the serving cell. It is applicable both when priorities and when offsets are used for cell reselection. Usually cell reselection is triggered before the S-criteria level is reached.
As an alternative to the signal quality evaluation as described above, the signal strength can also be used for cell reselection evaluation. Thus, the signal strength of serving cell is compared if it is below a threshold level (Threshserving,Low) and if that is the case the terminal is allowed to reselect a lower priority cell if the measured signal strength of the radio channel of the lower priority cell is above a threshold (Threshx,Low). Similarly, higher priority cells are reselected if signal strength of radio channel in the higher priority cell exceeds a threshold (Threshx,High). Then acceptable signal quality of serving cell may be checked by S-criteria.
Utilization of either measure indicating signal strength or a measure indicating signal quality as a characteristic of a radio signal may cause serious problems. For example, in UTRAN the CPICH RSCP gives information on the coupling loss, but does not say anything on the interference. On the other hand, CPICH Ec/No is a composite measure of coupling loss and downlink interference. Furthermore, it has been found out that CPICH RSCP is a good measure of uplink quality and similarly CPICH Ec/No is a good measure for downlink quality in UTRAN. Thus, if either of the measures is used only, it is possible that either the uplink or downlink connection fails when initiating a connection even if the link in the opposite direction experiences no problems.
More specifically, the same problem exist both when measuring the characteristics in the serving cell or in any neighboring cells. In case of measurement of the characteristics of the serving cell it may cause the mobile terminal to stay camped too long time to that specific cell due to the misinterpretation of the general quality of the cell based only on one character. Similarly, when the terminal is reselecting a target cell, and if the reselection is based only on one character relating to a radio channel of the target cell, it may cause the terminal to reselect such a cell that has very bad link to either of the directions, which was not measured.
In the worst case the mobile terminal may end up to a situation, in which continues ping-pong cell reselections happen i.e. the mobile terminal selects back and forth between the same cells due to faulty measurement results and their analysis. A concept called hysteresis is utilized for avoiding the ping pong effect. The hysteresis is how much a radio channel measure on serving cell has to increase and/or on target cell has to decrease after reselection to target cell to make the serving cell allowed for cell reselection again. The amount of hysteresis shall be defined by means of threshold selection in such a manner that the mobile terminal gets enough candidates for target cells as a result of measurements, but at the similar time the risk for ping pong is minimized. More specifically, by increasing the hysteresis the reselection rate may be reduced. On the contrary, for a given measurement time interval called treselection, there is an optimal amount of hysteresis that maximizes cell quality by preventing the UE from making wrong reselection decisions in case of large signal fluctuations (as expected higher hysteresis is required for smaller values of Treselection). Utilization of either measure indicating signal strength or a measure indicating signal quality as a characteristic of a radio channel to determine when to reselect to target cell gives a hysteresis only on one measure. When the opposite measure is closed to S-criteria a small change in this measure may lead to reselection back and forth.